Rare earth element-iron-boron permanent magnet and method for the manufacture thereof

ABSTRACT

In a method for manufacturing a permanent magnet, a powder of a magnetic base alloy and powders of first and second binder alloys are mixed. The magnetic base alloy has a general formula SE 2  T 14  B, wherein SE is at least one rare earth element, including Y, and T is Fe or a combination of Fe and Co, wherein Co does not exceed 40 wt % of the combination of Fe and Co. Each of the first and second binder alloys has a general formula SE a  Fe b  Co c  B d  Ga e , wherein 15&lt;a&lt;40, 0&lt;b≦80, 5≦c≦85, 0&lt;d≦20, 0&lt;e≦20, and a+b+c+d+e=100, and wherein the second binder alloy contains approximately 2.5 wt % fewer rare earth elements and approximately 1.5 wt % less gallium compared to the first binder alloy. The base alloy and the binder alloys are mixed in a weight ratio of base alloy to binder alloys between 99:1 and 90:10, and is subsequently compressed and sintered in a vacuum and/or in an inert gas atmosphere.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a permanent magnet of the typeSE-Fe-B that has the tetragonal phase SE₂ Fe₁₄ B as the principal phase,wherein SE is at least one rare earth element, including Y.

2. Description of the Prior Art A magnet of the above general type isdisclosed, for example, in European Application 0 124 655 and in U.S.Pat. No. 5,230,751 that corresponds therewith. Magnets of the typeSE-Fe-B exhibit the highest energy densities currently available.SE-Fe-B magnets manufactured by powder metallurgy contain approximately90% of the hard-magnetic principal phase SE₂ Fe₁₄ B.

U.S. Pat. No. 5,447,578 also discloses SE-Fe-B magnets that containSE-Fe-Co-B-Ga phases as admixtures.

One usually proceeds such in the manufacture of such SE-Fe-B magnets bymixing a SE-Fe-B base alloy with the a composition close to the SE₂ Fe₁₄B phase and a binder alloy with a lower melting temperature. The goal isto set the structure of the SE-Fe-B sintered magnets of SE₂ Fe₁₄ B basealloys with inter-granular binders, while using optimally little binderalloy.

European Application 0 517 179 proposes the employment of binder alloyshaving the composition Pr20Dy₁₀ Co₄₀ B₆ Ga₄ Fe_(rest) (in weightpercent, this is Pr≈35, Dy≈20, Co≈28, B≈0.77, Ga≈3.5).

The special characteristic of this Pr20Dy₁₀ Co₄₀ B₆ Ga₄ Fe_(bal) binderalloy is that it is composed of four inter-metallic phases. SEMinvestigations have documented that all four existing principal phasescontain B and Ga. These, namely, are phases of the types:

SE₅ (Co, Ga)₃

SE(Co[sic], Fe, Ga)₂,

SE(Co, Fe, Ga)₃

SE(Co, Fe, Ga)₄ Bx.

The melting temperatures of the phases lie at approximately 560° C.,980° C., 1060° C. and, respectively, 1080° C. The phase 1/3 and 1/4boride in fact have relatively high melting temperatures, but it isimportant that these lie just below the sintering temperature or,respectively, that they become molten at the sintering temperature. Thephases 1/2, 1/3 and the 1/4 boride are ferromagnetic or ferrimagneticwith Curie temperatures of 110° C., 340° C. and, respectively, 375° C.

It has now turned out that the proportion of this binder alloy in themixture of the base alloy must lie within 7-10 weight %. In this mixingrange, sinter densities of approximately ρ>7.55 g/cm³ are achieved onlyat sintering temperatures above 1090° C. These sinter densities roughlycorrespond to 99% of the theoretical density. Outside this mixing range,the sinterability and, thus, the remanence that can be achieved areconsiderably influenced. The grain growth is highly activated in themagnets with a proportion of this binder alloy of more than 10 weight %,but the pores are not closed. The consequence is the formation of astructure with anomalously large grains (>50 μm) and with high porosityas well as with low sinter densities. Given lower proportions of binderalloy, the amount of the fluid phase is accordingly not adequate for thedensification.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide apowder-metallurgical manufacturing method for permanent magnets of theSE-Fe-B type that exhibits an enhanced sinterability compared to theknown methods as well as a very good remanence.

The object is inventively achieved by a method that comprises thefollowing steps:

a₁) a powder of a base alloy of the general formula

    SE.sub.2 T.sub.14 B,

wherein SE is at least one rare earth element, including Y, and T is Feor a combination of Fe and Co, wherein the Co part does not exceed 40weight % of the combination of Fe and Co,

a₂) and a powder of a first binder alloy of the general formula

    SE.sub.a Fe.sub.b Co.sub.c B.sub.d Ga.sub.e

and a powder of a second binder alloy of the general formula

    SE.sub.2 Fe.sub.b Co.sub.c B.sub.d Ga.sub.e

wherein SE is at least one rare earth element, including Y, with15<a<40, 0<b≦80, 5≦c≦85, 0<d≦20, 0<e≦20 under the conditiona+b+c+d+e=100, whereby the second binder alloy contains approximately2.5 weight % fewer rare earth elements and approximately 1.5 weight %less gallium compared to the first binder alloy, are mixed in a weightratio of base alloy to binder alloys of 99:1 to 90:10;

b) the mixture is compressed and, subsequently,

c) is sintered in a vacuum and/or in an inert gas atmosphere.

It has been shown that permanent magnets manufactured in this wayexhibit very high remanences, and that the proportion of binder alloycompared to the proportion of the base alloy can be reduced to below 7weight %. Further, the additional gallium-containing phase of the binderalloy exhibits especially good wetting properties.

DESCRIPTION OF THE DRAWINGS

The single FIGURE shows typical demagnetization curves for magnetsmanufactured in accordance with the inventive method and having theinventive composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is explained in greater detail below on the basis of theexemplary embodiments and the FIGURE. A Nd₂ Fe₁₄ B base alloy (Table 1a)and two binder alloys (Table 1b) with the following compositions wereemployed for the investigation:

                  TABLE 1a                                                        ______________________________________                                        Composition (weight %)                                                          Melt     Nd      Pr   Dy    SE   B     Al    Fe                             ______________________________________                                          SV 94/84 28.1 0.08 <0.01 28.2 1.01 0.03 Bal.                                ______________________________________                                    

                  TABLE 1b                                                        ______________________________________                                        Ga                                                                              Concentration Composition (weight %)                                        Melt   (At. %) (Wt. %) Pr   Dy   Co   B    Ga   Fe                            ______________________________________                                          SV 94/86 3.1 2.65 36.3 20.5 25.1 0.77 2.65 Bal.                               SV 94/108 1 ˜1 33.85 19.6 28.25 0.75 1.05 Bal.                        ______________________________________                                    

The following mixtures were prepared from coarse powders of thesealloys.

                  TABLE 2                                                         ______________________________________                                              G.L. (SV 94/84)                                                                            B.L. (SV 94/86)                                                                            B.L. (SV 94/108)                                Mixture (Wt. %) (Wt. %) (Wt. %)                                             ______________________________________                                          295/1 90 10 --                                                                295/2 90 6.66 3.33                                                            295/3 90 3.33 6.66                                                            295/4 90 --  10                                                             ______________________________________                                        The calculated composition of the manufactured magnets then yield:             Composition in Weight %                                                        SE        Dy     Pr     B    Co     Ga   Fe                                 ______________________________________                                          31.05 2.05 3.65 0.986 2.51 0.265 Bal.                                         30.9 2.6 3.55 0.985 2.6 0.21 Bal.                                             30.8 1.97 3.65 0.985 2.7 0.155 Bal.                                           30.7 1.96 3.4 0.984 2.8 0.105 Bal.                                          ______________________________________                                    

The mixtures were finely ground in a planetary ball mill or 120 minutes;the average particle size of the fine powder achieved 2.4 μm.Anisotropic, isostatically pressed magnets were manufactured from thefine powders. They were sintered to densities of ρ>7.50 g/cm³ andsubsequently tempered.

The magnets were sintered as follows:

1090° C./34 (1 h vacuum+2 h in argon)

1070° C./34 (1 h vacuum+2 h in argon)

1060° C./34 (1 h vacuum+2 h in argon)

Extremely high sinter densities of ρ>99% were already measured atsintering temperatures of 1060° C.

The typical demagnetization curves of the magnets are shown in theFIGURE. At room temperature, the magnets achieve remanences of 1.39 to1.41 T and coercive field strengths H_(cJ) >14 kOe. The magnets achievea very high alignment of the grains (98-98.6%)

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

We claim as our invention:
 1. A method for manufacturing a permanentmagnet, comprising the steps of:a) mixing a powder of a magnetic basealloy of the general formula

    SE.sub.2 T.sub.14 B,

wherein SE is at least one rare earth element, including Y, and T isselected from the group consisting of Fe and a combination of Fe and Co,wherein Co does not exceed 40 weight % of the combination of Fe and Co,and a powder of a first binder alloy of a general formula

    SE.sub.a Fe.sub.b Co.sub.c B.sub.d Ga.sub.e

and a powder of a second binder alloy of a general formula

    SE.sub.a Fe.sub.b Co.sub.c B.sub.d Ga.sub.e

wherein 15<a<40, 0<b≦80, 5≦c≦85, 0<d≦20, 0<e≦20 and a+b+c+d+e=100, andwherein the second binder alloy contains approximately 2.5 weight %fewer rare earth elements and approximately 1.5 weight % less galliumcompared to the first binder alloy, in a weight ratio of base alloy tobinder alloys between 99:1 to 90:10 to obtain a mixture; b) compressingthe mixture to obtain a compressed mixture; and c) sintering thecompressed mixture in an environment selected from the group consistingof a vacuum and an inert gas atmosphere.
 2. A method according to claim1, wherein the step of mixing comprises mixing said base alloy and saidbinder alloys in a weight ratio of base alloy to binder alloys between99:1 and 93:7.